Study of the metabolites produced by bacteria which inhibit vitamin B12 promoted growth response of a mutant culture of Escherichia coli has shown that several amino acid and peptides have this capability. Several of these are not toxic to mammalian cells in tissue culture and yet apparently do block the absorption of vitamin B12 by the bacteria and by the B12 binding proteins in serum. One of these substances, a peptide, inhibits methylation promoted by vitamin B12 coenzymes.